Most content has a shared page with the new FAKE 5 modules or was integrated in the appropriate module.
For example topics about "Targets" like "specifictargets" has been added to the "Core.Targets" module.
You can find a link to the old documentation at the top of the new documentation.

So to find the old documentation you usually:

Find the module in the navigation

Navigate to the FAKE 4 docs (linked at the top in bold)

NOTE: This page might be removed without further notice once FAKE 5 has been released!